Hormone replacement therapy and the risk of breast cancer.

Hormone replacement therapy (HRT) is given to relieve the climacteric symptoms of menopause. Use of HRT reduced after a report from the Women's Health Initiative linked it to an increased risk of breast cancer. This association has been confirmed in several other studies, including the Million Women Study. The risk of breast cancer is greater for formulations that contain both estrogen and progesterone, compared with estrogen alone. The breast cancer risk associated with HRT is higher for estrogen receptor-positive cancers than for estrogen receptor-negative cancers, and for low-grade cancers compared with high-grade cancers. After cessation of HRT the increased risk of breast cancer dissipates within 2 years. The rapidity of the decline suggests that a proportion of breast cancers that are hormone dependent will regress if the hormonal stimulation is removed. In evaluating a woman who is considering HRT, factors that have been associated with an increased risk include the initiation of hormone use immediately after menopause, a lean body mass and high mammographic breast density.